1. Field of the Invention
This invention relates to a semiconductor laser having a doubleheterostructure in which cladding layers are formed on each of both faces of an active layer.
2. Description of the Prior Art
To shorten oscillation wavelength from a semiconductor laser is advantageous in that the diameter of focused beams in an optical disk can be reduced, thereby increasing the amount of information that can be written therein, and the depth of focus of an optical disk can be increased. Also, it is advantageous in that the sensitivity of a laser beam printer can be improved due to an improved photosensitivity curve of the photoconductive member therein.
However, as the oscillation wavelength from a semiconductor laser, in which the lattice constant of a substrate crystal coincides with the lattice constant of each of all layers successively formed on the substrate crystal, is shortened, the energy difference between the direct transition and the indirect transition within the active layer becomes small thereby increasing the distribution proportion of current carriers injected into the direct transition region to the indirect transition region, resulting in an increase in the threshold current level of the semiconductor lase. Moreover, the energy difference between the active layer and the cladding layers becomes small, so that current carrier injected into the active layer flow to the cladding layers resulting in an increase ni the threshold current level, as well.
On the contrary, in a semiconductor laser in which the lattice constant of a substrate crystal does not coincide with the lattice constant of each of all layers successively formed on the substrate crystal, the energy difference therebetween can be maintained at a high level, while crystal defects result from the uncoincidence between the lattice constants so that the threshold current increaases and/or the lifetime is reduced.